Receptor antibodies have been used in a new radioimmunoassay for insulin receptors, which is being applied to study receptor phylogeny and function. A fraction of insulin non-dissociable from cells after binding has been shown to be covalently coupled to insulin receptors. Immunoprecipitation of receptors has been used as an assay to screen for receptor antibodies. Low titer IgM antibodies have been found in patients with diabetes and in the New Zealand obese (NZO) mouse - a model of human adult-type diabetes. The NZO mice also have evidence of generalized autoimmunity with antibodies to DNA and dense deposits of IgM in the renal golmeruli. Their renal pathology resembles that of both systemic lupus erythematosis and diabetes mellitus.